How do you know if certain variables in physics are 0

In some physics questions "vi", "vf", or "a" are equal to 0. How do you know if they are or not?

If the object starts at rest, the initial velocity (vi) is 0.
If the object comes to a stop, the final velocity (vf) is 0.
If the object has a constant speed, the acceleration (a) is 0.

Cheers!